The most common disease of the heart is coronary artery disease (CAD). With coronary artery disease, an insufficient amount of blood flows to the heart muscle which contracts to produce the pumping action. Increased exercise or shock requires the heart to pump faster in order to distribute more oxygenated blood to the body in a given time. Under such circumstances, the heart muscle needs increased blood flow to function properly. With the constricted blood flow due to the presence of CAD, the heart muscle does not get enough blood and malfunctions in either a continual or intermittent fashion under stressful circumstances, while performing adequately under non-stressed conditions. The malfunction may manifest itself as a reduced ability to contract the heart chambers sufficiently to adequately increase blood flow. The amount of reduction indicates the degree or severity of the disease. An intermittent malfunction, known as an ischemic event, can be very brief, e.g. a few heart beats, or can be longer in more progressed states of disease. The term heart beat, as used herein, refers to one complete pumping cycle of the heart.
The effectiveness of detecting heart malfunctions and diagnosing CAD can be rated in terms of sensitivity and specificity. Sensitivity is a measure of a particular diagnostic ability to detect an event related to a heart malfunction. For example, a method with higher sensitivity is more likely to detect an ischemic event. Specificity is a measure of the ability to pinpoint the exact cause and location of the ischemic event, e.g. seventy percent blockage of the left coronary artery. It is desired that the initial tests used for diagnosing heart disease be as sensitive as possible to check for the presence of CAD, while tests pertaining to specificity may be more intensive after the disease has been detected.
One widely used technique for monitoring heart performance is an electrocardiogram (EKG), used in conjunction with a stress test. A stress test normally involves monitoring the heart performance before, during and after the patient experiences a drug-induced heart rate increase or performs a controlled exercise, e.g. cycling on a stationary bicycle or running on a treadmill. The stress test attempts to create exercise and stress conditions during which a diseased heart will not function properly. The EKG displays or records the electrical nervous signals supplied to the heart to cause it to beat. However, the EKG does not convey significant information regarding the movement or performance of the heart muscles in response to these signals.
A significant improvement in the ability to detect CAD resulted from the use of ultrasound imaging devices to observe a moving muscle wall of the heart. By definition, ultrasound is a sound pressure wave signal having a frequency greater than twenty kilohertz. An ultrasound transducer, which converts an electrical signal to a transmitted ultrasound wave and then converts the reflected ultrasound wave back to an electrical signal, is placed over the area of interest in a patient's body. In the case of diagnosing heart disease, the area of interest is, for example, the base or apex of the heart for observation of segments or sections of the left ventricle. A two-dimensional image is generated by sweeping the transmitted ultrasound wave through a fixed angle, the reflections from which are processed as a wedge or sector shaped image. The image generated is essentially a real-time image since the delays involved in receiving and processing the reflected ultrasound wave into the image therefrom are negligible.
Further improvements in the field of using ultrasound to monitor heart performance involved the addition of a recording capability, in order to preserve the ultrasound images. Videotape was used, as well as a device known as a Video Sheet Recorder (VSR), which records continuously for up to 10 seconds onto a disc shaped pieced of magnetic storage material similar to the magnetic tape used in video tapes. The advantage of a video sheet over a video tape is the ease with which a portion of the sheet can be replayed, since no rewinding is necessary. With both of these recording devices, images of the heart are continuously recorded in all states or phases of the heart beat cycle rather than synchronized to a particular point or points in the cycle. Continuous recording of the images does not readily lend itself to the type of comparisons which are advantageous enough for detecting CAD with a high degree of sensitivity. Since CAD manifests itself by a reduced compression effect of the heart chambers, it is desirable to observe and compare a series of images of the compression segments from successive heart cycles for successful diagnosis. Since the continuous, non-synchronized images obtained over the relatively short time period of ten seconds makes it difficult to compare successive heart cycles at similar conditions over a sufficient time to detect ischemic events, the sensitivity of the VSR device has not generally been regarded as sufficient for adequate detection of CAD.
A further improvement to the field of using ultrasound to monitor heart performance was realized in a device which senses the electrical nerve signal sent to the heart and uses this signal to synchronize the acquisition of images. A series of images from predetermined points in each of four successive heart beats is acquired and temporarily preserved. Eight images, generally equally spaced apart in time, are obtained of each heart beat cycle beginning with an image of the heart in the diastolic state and ending with an image of the heart in the systolic state. This series of images from one heart beat cycle represents the performance during compression of the heart chamber and is called an image loop. A display showing four images simultaneously (a quad-image) provides corresponding images of the same synchronized points from the preceding four heart cycles. After acquiring the four images, the operator quickly reviews the four heart beat cycles. This is done by scanning through the four image loops which are displayed in quad-image format, resulting in the simultaneous display of the corresponding images of the four image loops from the previous four heart beats. If desired, the operator may record the quad-image group in memory. The operator then repositions the sensor, if necessary, and acquires another quad-image group which must be similarly reviewed, and so on. While the transducer is removed and/or the four acquired heart beat cycles in the quad-image group are reviewed, the heart continues to beat and image loops of these beats are not acquired or observed. If an ischemic event occurs during this time, it is missed and not recorded. If the operator acts quickly and with facility to review the images, to record them, and to reposition the sensor in two seconds, only a few beats are missed, e.g. five, assuming a heart rate of one hundred-fifty beats per minute. Therefore, at about the quickest usage rate possible, image loops of four heart beats are acquired for every five heart beats for which no image loops are acquired. The number of non-acquired image loops becomes even greater with less accomplished operators who take longer times between acquisitions. Since an ischemic event may occur randomly and intermittently even under stress conditions, there is a significant possibility that previous devices may fail to provide the necessary sensitivity to detect CAD or other heart disease.
It is with regard to these and other considerations, and the desire to increase the sensitivity or likelihood of detecting ischemic events which are indicative of CAD and other heart disease, that the present invention has resulted.